Hypotheses

What is a Hypothesis?

A hypothesis is an "educated guess." It can be an educated guess
about what nature is going to do, or about why nature does what it
does.

"Hypotheses are single tentative guesses--good
hunches--assumed for use in devising theory or planning
experiment, intended to be given a direct experimental test when
possible." (Eric M. Rogers, "Physics for the Inquiring Mind."
(Princeton University Press, Princeton, NJ, 1966)

What makes a statement a
scientific hypothesis, rather than just an interesting
speculation? A scientific hypothesis must meet 2 requirements:

A Scientific Hypothesis Must Be "Testable".

Science proceeds by making observations of nature (experiments).
If a hypothesis does not generate any observational tests, there is
nothing that a scientist can do with it. Arguing back-and-forth about
what should happen, or what ought to happen, is not the way science
makes progress.

Consider this hypothesis:

Hypothesis A:

"Our universe is surrounded by another, larger universe, with
which we can have absolutely no contact."

This statement may or may not be true, but it is
not a scientific hypothesis. By its very nature it
is not testable. There are no observations that a scientist could
make to tell whether or not the hypothesis is correct. Ideas such as
Hypothesis A are interesting to think about, but science has nothing
to say about them. Hypothesis A is a speculation,
not a hypothesis.

Often the requirement that a scientific
hypothesis must be testable is phrased as "a scientific hypothesis
must generate predictions". The word "predictions"
can often cause confusion, since we commonly think of a prediction as
telling about something that is going to happen in the future,
like "Next year, Lindsay Lohan will marry a frog." A scientific
prediction is not
something that is going to happen, but rather something that is
happening right now, but no one has ever noticed. In other words,
a prediction suggests a test (observation or experiment) for the
hypothesis. To say that a hypothesis "generates predictions" means
the same thing as saying the hypothesis "is testable".

A Scientific Hypothesis Must Be
"Falsifiable".

A
scientific hypothesis must be testable, but there is a much stronger
requirement that a testable hypothesis must meet before it can really
be considered scientific. This criterion comes primarily from the
work of the philosopher of science Karl
Popper, and is called "falsifiability".

Consider this hypothesis:

Hypothesis B:

"There are other inhabited planets in the universe."

This hypothesis is testable, but it is
not a scientific hypothesis. Here's why. Hypothesis
B may be either correct or wrong. If it is correct, there are several
ways that its correctness can be proven, including:

A space probe sent from earth to explore the universe sends
back the news that it has discovered an inhabited planet. (This
news is later confirmed by other space probes.)

Radio telescopes on earth begin to receive signals from
somewhere in the Andromeda Galaxy that appear to be reruns of the
"I Love Telek" show.

Knock, Knock. "Greetings, earthling! I am Telek from the
planet Zoron in the Andromeda Galaxy. I have just landed in your
backyard. Take me to your leader."

So, if Hypothesis B is true, there are observations that
scientists could make that would prove its correctness. But, the
hypothesis may be wrong. (Most hypotheses are...) If
Hypothesis B is wrong, there is no test that will prove it.
If one of our space probes never finds an inhabited planet, it
doesn't mean that one doesn't exist. If we never receive signals from
space, or Telek never lands in your back yard, that does not prove
that the hypothesis is wrong, either. Hypothesis B is not
falsifiable.

What about this:

Hypothesis C: "Any two objects dropped from the
same height above the surface of the earth will hit the ground at the
same time, as long as air resistance is not a factor."

Hypothesis C is a scientific hypothesis
because:

It is testable - pick 2 objects, and drop
them. Of course, you may have to provide a vacuum for them to fall
in, in order to remove air resistance from consideration.

It is falsifiable - If anyone finds 2 objects
that don't hit the ground at the same time and can show that it is
not due to air resistance, then she has proven the hypothesis
wrong. This hypothesis "sticks its neck out" for every test. In
theory and in practice, if Hypothesis C were wrong, it would be
very easy and straightforward to show it.

In his youth, Karl Popper studied the "social theory" of Karl Marx and the "psychological theory" of Sigmund Freud. Both of these ideas claimed a scientific basis, and both could produce evidence to support their hypotheses - historical evidence on the part of Marx, and clinical case studies on the part of the Freud. Popper eventually became unhappy with both Marx and Freud (and their followers) because he felt that they were both too quick to "explain away" any evidence that contradicted their ideas. For instance, Marx had predicted that the communist revolution would begin in a highly industrialized country, like Britain or Germany. Instead, the communist revolution occurred in Russia, which was hardly industrialized at the time, and never spread to the industrialized nations. Marx's followers explained this by claiming that it was due to "unforseen historical accidents" and Marx wasn't actually wrong. Popper also noted that Freud often used essentially the same explanation to explain vastly different behavior - a brutal murderer was acting under the same influences as a generous philanthropist.

In contrast to this, Popper admired Albert Einstein and his Theory of Relativity. Einstein said, in effect (among other things), "If you look at stars near the Sun during a total eclipse, you should observe a specific behavior. If this doesn't happen, my theory is wrong." Popper felt that this contrasted sharply with the ideas of Marx and Freud - Einstein was willing to "stick his neck out". Popper felt that this was the essence of a real scientific hypothesis.1

As Popper pointed out, it is relatively easy to gather evidence
for just about any idea, but a hypothesis is essentially worthless
unless it is "risky" - it must make predictions that could contradict it. The process of gaining real confidence in a hypothesis, then, is not in accumulating evidence in its favor, but rather in showing that situations that could establish its falsity don't, in fact, happen.

Most Scientific Hypotheses Can't Be Proven Correct!

Note that it is very easy to prove Hypothesis C wrong (if it
were), but it is impossible to prove it
correct! Since Hypothesis C states that any pair of
objects behaves in a certain way, in order to prove it correct, all
possible combinations of objects that exist (or have ever, or will
ever exist) must be tested. This is clearly not possible. As we test
Hypothesis C more and more, we can get more and more confident in its
truth, but we can never be absolutely sure. Someone could always come
up with 2 objects tomorrow which don't behave exactly as Hypothesis C
says they should, and this would make Hypothesis C incorrect.

Actually, this almost happened. Just a few years ago a group of
physicists published a paper claiming that careful reanalysis of some
experimental data published at the turn of the century (which
confirmed Hypothesis C) actually showed that things made of large,
heavy atoms fall very slightly faster than things made of small,
light atoms. This "fifth (antigravity) force" idea caused quite a
stir for a short while, but no one has (so far) been able to confirm
this effect. If other physicists had been able to observe it,
Hypothesis C would have been proven wrong.

Wimping Out:

It sometimes bothers people that scientific facts, hypotheses,
laws, and theories generally can't be proven to be true. It generally
doesn't bother scientists, however. You might say, "Can't scientific
hypotheses be phrased so that they could be proven true?" For
example, why not:

Hypothesis D:

"This big object right here and this little object right there
will hit the ground at the same time when I drop them from the same
height."

Hypothesis D is a scientific hypothesis - it is testable, and it
is falsifiable. There are two problems with it, however:

It is a very wimpy hypothesis. Compared to Hypothesis C, which
is quite powerful and useful, Hypothesis D is practically useless,
and;

Hypothesis D can't be proven correct, either!
Who is to say that someone won't show up tomorrow with some brand
new, super-sophisticated, high-tech measuring instrument and say
"Look! My measuring device clearly shows that the little object
hits the ground fully a half a trillionth of a second before the
big one." The best we can ever say (as scientists) is something
like, "It certainly appears to me at this time that both objects
hit the ground at the same time." Even timid little Hypothesis D
cannot be proven to be absolutely true!

What if the Hypothesis Fails a Test?

If a hypothesis fails a test, it cannot be true, and it must be
modified or discarded. In science, if there is a conflict between
observation and hypothesis, the hypothesis loses. It doesn't matter
whose hypothesis it is or how famous they are - if the hypothesis
does not conform to reality it must be rejected.

Occam's Razor:

What if two or more competing hypotheses both pass some initial
tests - how do you choose between them?

Certainly, if the hypotheses generate different predictions it
will be a simple matter to pick the best one - as long as it is
feasible to carry out the experimental tests. What if the competing
hypotheses don't give distinguishable, feasible predictions? Enter
"Occam's Razor".

William of Occam was a medieval scholar and logician, and, in
modern form, the principle that has come to be known as Occam's Razor
says:

If two hypotheses can't be distinguished
experimentally, choose the simpler one.

Where do Hypotheses Come From?

What procedure or formula do scientists use to generate
hypotheses? There isn't one. Generating hypotheses is a creative
process. It takes knowledge, experience, skill, intuition, and
creativity to come up with a great hypothesis, just as it takes
knowledge, experience, skill, intuition, and creativity to paint a
great picture or compose a great symphony. In the words of Sir Peter
Medawar:

"The truth is not in nature waiting to declare itself,
and we cannot know a priori which observations are relevant and
which are not: every discovery, every enlargement of the
understanding begins as an imaginative preconception of what the
truth might be. The imaginative preconception--a
'hypothesis'--arises by a process as easy or as difficult to
understand as any other creative act of mind; it is a brain-wave,
an inspired guess, the product of a blaze of insight. It comes,
anyway, from within and cannot be arrived at by the exercise of
any known calculus of discovery. A hypothesis is a sort of draft
law about what the world--or some particularly interesting aspect
of it-- may be like; or in a wider sense it may be a mechanical
invention, a solid or embodied hypothesis of which performance is
the test." (P. B. Medawar, Advice to a Young Scientist (Harper and
Row, New York, 1979), p. 84.)

By the way...

Just because a hypothesis is not scientific does not mean that no
scientist will ever investigate it. Hypothesis B, for instance, is
just too "juicy" for some people to pass up. It is currently being
investigated by scientists (and has been for many years), with such
programs as "SETI"
(Search for Extra-Terrestrial Intelligence) which uses large radio
telescopes to try to detect signals from outer space (Test 2 on page
1). A lot of time, effort, and money has been spent on it. Hypothesis
B is not a falsifiable scientific hypothesis - so such an
investigation can only pay off if the hypothesis is correct and the
telescopes detect something recognizable. So, is the possible payoff
worth the effort and expenditure? This is a question that you will
need to answer for yourself. As a taxpayer, after all, it's your
money!

Practice quiz on scientific hypotheses.

1The ideas in the previous two paragraphs are from the fascinating course "Philosophy of Science" by Jeffery L. Kasser, published by The Teaching Company